1. Field of the Invention
The invention generally relates to a power conversion field, in particular, to a bootstrap gate driver applied in a synchronous rectification circuit/application.
2. Description of Related Art
There are two types of rectification strategies in the power conversion field, one type is diode rectification, and the other type is synchronous rectification. Diode rectification is performed based on the conduction of diodes. Synchronous rectification is performed by usually using a gate driving signal to control the operation (i.e. turned on or off) synchronous rectifiers (SRs). Synchronous rectifiers are usually implemented by MOSFETs. For low-voltage and large-current power conversion application, the forward bias (Vf) of diodes cannot be further reduced due to the characteristic of diodes, so the power loss is higher; and on the other hand, the on-resistor (Rds-on) of each synchronous rectifier is smaller, so the power loss is lower compared with diode rectification. Therefore, the conversion efficiency of the power conversion can be increased due to the power loss of synchronous rectifiers is lower.
There are conduction loss, driving loss, switching loss and body diode loss in the operating region of synchronous rectifiers. When the load is in the heavy-loading state, the load current becomes larger and makes the conduction loss become the main part of loss. When the load is in the light-loading state, the load current is small, such that the switching loss and driving loss both caused by turning on/off synchronous rectifiers become the main part of loss.
Compared with diode rectification, the saved power of synchronous rectification is lower, and the power loss related to synchronous rectification is even higher than that of diode rectification. In this case, a switching of synchronous rectification needs to be stopped/inactivated, and the body diode of the corresponding synchronous rectifier is used to perform diode rectification. Moreover, the body diode can cut off the negative reactive current loop when the load is in the light-loading state, such that inductive loss and driving loss will be significantly reduced, and the light-loading efficiency will be obviously increased.
However, the current synchronous rectification may be implemented by a couple of synchronous rectifiers to be respectively served as a high-side N-type transistor functioned as a rectifier and a low-side N-type transistor functioned as a switch. And, as shown in FIG. 1, a traditional bootstrap gate driver 101 is used to drive the high-side N-type transistor QH and the low-side N-type transistor QL in response to an inputted pulse width modulation (PWM) signal PWM_I. Since the high-side N-type transistor QH and the low-side N-type transistor QL are alternately switched by the bootstrap gate driver 101 in response the inputted PWM signal PWM_I, so the high-side N-type transistor QH can not be disabled when the load is in the light-loading state by directly turning off the inputted PWM signal PWM_I due to both high-side N-type transistor QH and the low-side N-type transistor QL are commonly used the inputted PWM signal PWM_I (if the inputted PWM signal PWM_I is turned off, the operation of the low-side N-type transistor QL is affected). In other words, a switching of the synchronous rectification is still activated when the load is in the light-loading state, such that the light-loading efficiency will not be effectively increased in case that the traditional bootstrap gate driver 101 is applied in the synchronous rectification application.